This application claims the priority of German patent application 101 01 992.0, filed Jan. 18, 2001, the disclosure(s) of which is (are) expressly incorporated by reference herein.
The invention relates to a ROSAR (rotating synthetic apparatus radar method for landing helicopters under adverse weather conditions, and for recognizing and detecting concealed targets.
A ROSAR device, described in German patent documents DE 39 22 086 and DE 43 23 511 can be used in near real time on-line operation and is able to perform, in addition to mapping, obstacle warning, mine detection, landing assistance, reconnoitering and tracking as well as missile guidance functions. The ROSAR device is used in pulse frequency or FM-CW operation and in the cm- or in the mm-wavelength range.
With the current state of the art, due to the lack of running time-dependent range data, the creation of a so-called xe2x80x9cradar with vertical viewxe2x80x9d for SAR and ROSAR systems can only be realized using a costly phased-array system. In particular, the penetration of trees, shrubs, and other covered-up objects, such as flat layers of earth, require large wavelengths; and the desired resolution can be achieved only with very large dimensioned antennas. The latter, however, results in an extreme degradation of the flight performance and considerable expense.
It is an objective of the present invention to image the area that lies vertically below the helicopter at high resolution and to describe an evaluation procedure that does not rely on a wavelength-dependent range resolution. Another object of the invention is to provide such a procedure in which especially the horizontal resolution is not dependent on the depression angle. Finally, still another object of the invention to eliminate the disadvantages of the current state of the art.
These and other objects and advantages are achieved by the ROSAR process and apparatus according to the invention, in which the resolution cells (hereinafter, xe2x80x9cpixelsxe2x80x9d) are characterized solely by their own xe2x80x9cdoppler historiesxe2x80x9d (that is, doppler frequency displacement). This is in contrast to a traditional ROSAR system which provides that a pixel is formed with regard to range and azimuth angle or that according to the principle of the real aperture a surface-type and fixedly mounted antenna detects the elevation and azimuth angles.
The invention thus permits the use of an already available ROSAR system, adding only a special software solution in the ROSAR processor. The large distance between the receiving antenna and the point of rotation of the rotor allows for a very high ROSAR resolution, even in connection with large wavelengths that are necessary for penetrating concealed targets, mines and people. The method according to the invention can be used in either a pulse frequency or FM constant wave context, and in the cm or mm wavelength range.
According to the invention, each target (pixel) in the area to be imaged is irradiated with radar waves by the transmitting antenna. A fraction of the transmitted radar wave is reflected by each pixel in the area to be imaged, towards the receiving antenna based on the backscattering cross-section of that pixel. It should be noted in this regard, that the area to be imaged is that area which is located directly below the helicopter. This area is characterized by concentric rings located under the rotational axis of the rotor blade of the helicopter with each ring comprising a plurality of pixels (FIG. 2a). The pixels are characterized in polar coordinates by the indication of angle and range.
It is known from the principles of synthetic aperture radar, that because of the movement of the receiving antenna relative to the pixel on the ground in the area to be imaged, the distance between the receiving antenna and the pixel changes continuously. Thus, the receiving signal shows a doppler frequency displacement compared to the transmitting signal, and, as noted previously, each pixel is characterized by a significant doppler frequency displacement (xe2x80x9cdoppler historyxe2x80x9d) and can be distinguished from adjacent pixels on the ground in the area to be imaged.
The reference signals of all pixels in the area to be imaged are stored in a reference signal memory, with the storage positions addressed by the angular position of the pixel. Because of the rotational symmetry of the position of the pixels, all pixels of a pixel ring have the same reference signal. In other words each pixel ring is defined by a single reference signal.
In the case of receiving, the area to be imaged is irradiated by radar pulses. The backscattered sum signal of all pixels in the area to be imaged is stored in the current receiving signal memory with the storage positions addressed by the current position of the rotor blade (FIG. 2b). This sum receiving signal is correlated with the reference signals of all pixel rings; that is, it is cross-correlated with the reference signal of each pixel ring. In other words the correlation of the sum receiving signal with the reference signal of all the pixel rings is performed step-by-step, whereby at each step a cross-correlation of the sum receiving signal with the reference signal of a single pixel ring is performed.
It is clear, that the number of cross-correlations to be performed is equal to the number of pixel rings. With performing the cross-correlation function by correlating the sum receiving signal with the reference signal of a pixel ring the degree of brightness of each pixel in the pixel ring is calculated. Thus, the pixels of the pixel ring characterized by its degree of brightness can be displayed on a monitor, i.e., a 2-d image of the area to be imaged.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.